In recent years, with progress in digital technology and an improvement in data compression techniques, optical disks such as DVDs (digital versatile disks) have drawn attention as media for recording information such as music, movies, photographs, and computer software (hereinafter also referred to as “contents”). As optical disks have become lower in price, optical disk units employing optical disks as media for recording information have become widely used.
In the optical disk unit, information is recorded on an optical disk by forming a minute laser light spot on the recording surface of the optical disk on which a spiral track or concentric tracks are formed, and information is reproduced from the optical disk based on reflected light from the recording surface. An optical pickup unit is provided in the optical disk unit in order to emit laser light onto the recording surface of the optical disk and receive reflected light from the recording surface.
In general, the optical pickup unit includes an optical system, a photodetector, and a lens drive unit. The optical system includes an objective lens. The optical system guides a light beam emitted from a light source to the recording surface of the optical disk, and guides a returning light beam reflected from the recording surface to a predetermined light-receiving position. The photodetector is disposed at the light-receiving position. The lens drive unit drives the objective lens in the directions of its optical axis (hereinafter also referred to as “focus directions”) and in the directions perpendicular to the tangential directions of the tracks (hereinafter also referred to as “tracking directions”). The photodetector outputs a signal including not only the reproduced information of data recorded on the recording surface, but also information necessary to control the position of the objective lens (servo information).
Information is recorded on the optical disk based on the length of each of a mark and a space different in reflectivity from each other, and their combination.
For example, when a mark is formed in rewritable optical disks such as DVD-RW (DVD-rewritable) and DVD+RW (DVD+rewritable) disks including a special alloy in their recording layers, the special alloy is rapidly cooled after being heated to a first temperature so as to be in an amorphous state. On the other hand, when a space is formed, the special alloy is gradually cooled after being heated to a second temperature (lower than the first temperature) so as to be in a crystalline state. As a result, the reflectivity is lower in the mark than in the space. Such control of special alloy temperature is performed by controlling the light emission power of laser light. At the time of forming marks in particular, the pulse shape of light emission power is set based on a rule (method) concerning the pulse shape of light emission power, etc., called a write strategy, in order to reduce variation in heat distribution due to preceding and subsequent marks and spaces.
In the optical disk unit, at the time of recording, an optimum write (recording) power is obtained by performing test writing in a preset test writing area called PCA (Power Calibration Area) before writing information in order that a mark and a space of target length are formed at a target position on the optical disk (see, for example, ECMA-337 Data Interchange on 120 mm and 80 mm Optical Disk using +RW Format —Capacity: 4.7 and 1.46 Gbytes per Side, December 2003). This operation is called OPC (Optimum Power Control).
The contents tend to increase in quantity year by year, so that a further increase in the recording capacity of optical disks is expected. Providing multiple recording layers is considered as means for increasing the recording capacity of optical disks, and lots of efforts are being made to develop optical disks having multiple recording layers (hereinafter also referred to as “multilayer disks”) and optical disk units to access the multilayer disks. It is also important to obtain an appropriate write power in the multilayer disks, and a variety of proposals have been made regarding OPC (see, for example, Japanese Laid-Open Patent Application No. 2004-310995).
However, in rewritable multilayer disks, which are not yet commercially available, for example, higher recording rates may cause variations in recording quality even when recording is performed with an optimum write power obtained by OPC.